1. Field of the Invention
The present invention relates to apparatuses and methods for optimizing the operation of ion thruster arrays. Particularly, the present invention relates to apparatuses and methods for balancing the emission current of neutralizers in ion thruster arrays.
2. Description of the Related Art
Ion propulsion generally involves employing an ionized gas accelerated electrically across charged grids to develop thrust. The electrically accelerated particles can achieve very high speeds. The gas used is typically a noble gas, such as xenon. The principle advantage afforded by ion propulsion systems over conventional chemical propulsion systems is their very high efficiency. For example, with the same amount of fuel mass an ion propulsion system can achieve a final velocity as much as ten times higher than that obtainable with a chemical propulsion system. Although the principle of ion propulsion was established many decades ago, it has only relatively recently been implemented in practical applications.
The delay in development of practical applications may be due, in part to the fact that, although they are efficient, ion propulsion systems develop very low thrust when compared with chemical propulsion systems. This reality has narrowed the range of ion propulsion applications. However, ion propulsion is well suited for space applications where low thrust is often acceptable and fuel efficiency is critical. More and more ion propulsion is becoming a component of new spacecraft designs. Many spacecraft, including satellites as well as exploration vehicles, are presently making use of ion propulsion systems.
For example, ion thrusters are currently used for spacecraft control on some communications satellites. In a typical satellite ion thruster, thrust is created by accelerating positive ions through a series of gridded electrodes at one end of a thrust chamber. The electrodes, known as an ion extraction assembly, create thousands of tiny beams of thrust. The beams are prevented from being electrically attracted back to the thruster by an external electron-emitting neutralizer. The power processing unit (PPU) is the device which serves to provide electrical control and power to drive the ion thruster, including control of the emission current in the neutralizer cathode.
FIG. 1 is a schematic diagram of a power processor system 100 operating with two individual power processor units 102A, 102B (collectively referred to as 102), one for each ion thruster 104A, 104B (collectively referred to as 104) of a thruster pair. The ion thrusters 104 each include a thruster body 106A, 106B (collectively referred to as 106) and a neutralizer body 108A, 108B (collectively referred to as 108) which are each electrically driven by their respective PPU 102A, 102B.
The principle elements of a PPU 102A, 102B include the discharge power supply 110A, 110B (collectively referred to as 110) coupled to the anode of the thruster body 106A, 106B to provide ionizing power to the fuel (e.g., Xenon) and the screen power supply 112A, 112B (collectively referred to as 112) coupled to the discharge cathode of the thruster body 106A, 106B to drive the main beam. In addition, the PPU 102A, 102B includes a discharge heater supply 114A, 114B (collectively referred to as 114) coupled to the discharge cathode heater to heat it to a high temperature at startup and initiate electron emission for gas ionization and a keeper supply 116A, 116B (collectively referred to as 116) coupled to the discharge keeper to maintain electron emission for ionization after startup. The accelerator supply 118A, 118B (collectively referred to as 118) accelerates ions out of the thruster body 106A, 106B.
The PPU 102A, 102B also includes similar power supplies to drive the neutralizer 108A, 108B which serves to both charge neutralize and current neutralize the ion beam. The neutralizer heater supply 120A, 120B (collectively referred to as 120), coupled to the neutralizer cathode heater, heats it to initiate electron emission from the neutralizer. The neutralizer keeper supply 122A, 122B (collectively referred to as 122) maintains electron emission after startup. The Zener diodes 124A, 124B (collectively referred to as 124) allow the respective neutralizer cathodes to float at whatever potential is necessary to supply the correct electron emission to neutralize the positive ion beam of thrusters 104.
One critical element of an ion thruster is the neutralizer cathode. The neutralizer cathode, commonly a hollow cathode, is generally life limited by its operating temperature; the higher the operating temperature, the shorter the operating life of the cathode (and accordingly, the shorter the operating life of the thruster). Because the operating temperatures of such cathodes are directly proportional to their emission current, control of the emission current in the neutralizer cathode is important to maximizing the thruster's operating life. Consequently, in order to maximize operating life, a single PPU is provided to drive each ion thruster due to sensitivities such as the cathode emission current. Thus, a typical satellite using four ion thrusters (two pairs) requires four separate PPUs so that all four thrusters are capable of being turned on simultaneously. Consequently, this adds considerably to the mass required to drive the ion thruster array.
There is a need in the art for power processor systems for ion thruster arrays which are robust with optimized life. Further, there is a need for such systems to have reduced mass. As detailed hereafter, the present invention meets these and other needs.